Telephone system



June 8, 1937. e. N. SAUL TELEPHONE SYSTEM 4 Sheets-Sheet 1 Filed Feb.28. 1955' m Um lNl ENTOR 6.7V. SAUL A TTORNEY June 8, 1937. G. N. SAUL2,033,404

' TELEPHONE sYs TEM I I Filed Feb 28, 1935 4 Sheets-Sheet 2 u E Q lNlENTOR 6. N. SAUL ATTORNEY June 8,1937. G. N. SAUL 2,083,404

I TELEPHONE SYSTEM I A INVENTOR a. /v. SAUL A 7'TORNEV Ju ne 8, 1937.

G. N. SAUL TELEPHONE SYSTEM Iiiled Feb. 28, 1935 4 Sheets-Sheet 4 I lINVENTOR' G. N. SAUL ATTORVEY Patented June 8, 1937 UNITED STATES PATENTOFFICE TELEPHONE SYSTEM George N. Saul, Brooklynp-N. Y,, assignor toBell Telephone Laboratories, Incorporated, New York, N. Y., acorporation of New York Application February 28, 1935, Serial No. 8,591

6 Claims. (011179-43) This invention relates to telephone systems andmore particularly to toll switchboardcircuit ar rangements. One objectof the invention is to prevent annoying clicks in an operators receiver5 when she ceases ringing on a called line in case the talk key isoperated at the time.

Another object is to prevent ringing current incoming over the callingline from entering the answering operators receiver in case she shouldinsert the answering plug of a cord circuit in the calling line jackwhile the talking key of that particular cord circuit is operated.

In certain types of switchboard circuits, the operators telephone set isconnected to the cord 15 circuit through what is termed a hybridrepeating coil. Such a hybrid coil arrangement is shown in Fig. 4 of theU. S. patent to Dietze 1,709,900.

In the arrangement embodying the present inin vention the networks orimpedance N1 and N2 of Fig. 4 of the above noted Dietze patent arenormally connected across the respective ends of the coil and areremoved only when the respective ends of the cord circuit are connectedto line D3 jacks and the talk key is operated.

With this arrangement it will be evident that when the talk key isnormal the'repeating coil, with respect to the connected transmitter, isbalanced and is what is known as in an anti-sidetone condition, 1. e. acondition such that sounds affecting the transmitter will cause nocorresponding efiect in the receiver. At such times it is frequentlynecessary that a supervising operator, who may have her telephoneconnected in multiple with the switchboard operator, be

" able to converse with that operator, which under the condition abovedescribed would be impossible.

Another object of the invention therefore is to enable a supervisor totalk with the operator at such times, in systems'arranged as abovedescribed.

A feature of theinvention whereby the first object is attained residesin disconnecting the operators induction coil from the conductor of 45the cord over which ringing current is being transmitted, when the,ringing key is operated,

and delay the reconnection of the cord in that direction for an intervalsufficient to allow any remaining charge in the line to dissipate.

50 Another feature whereby the second object is attained resides in aswitching arrangement whereby in case the cord circuit talk key. isoperated at the time the answering'plug of the cord circuit is insertedin the jack of the line over 55 which ringing current is incoming theconnection of the operators induction coil to the cord circuit will bedelayed until theincoming -ring. ing current ceases.

Still another feature whereby the last object is attained resides in aswitching arrangement whereby when the supervisor connects her tele-'phone set in parallel with the set of the regular operator'i'n case notalk key at the position is operated, a circuit will be established todisconnect one of the bridged networks from across the operatorsinduction coil, thereby unbalancing the coil and permitting conversationbetween the two telephone sets.

The invention will be understood from the followingdescription of a tollswitchboard in accordance therewith and illustrated by the accompanyingdrawings Figs. 1 to 4, arranged in accordance with Fig. 5. Fig. 1 showsa portion of an incoming toll line essential to an understanding of theinvention; Fig. 2 shows a toll cordcircuit TC of the so-called sleeve orthird wire control type; Fig. 3 is a so-called position circuit. Thecord circuit of Fig. 2 and all other cord circuits of the position arearranged to be connected with Fig. 3. Fig. 4 is an operators telephonecircuit cooperating with the aforesaid cord and position circuits.

Figs. 1, 2 and 3' correspond to and are identical with similarlynumbered figures forming apart of the-disclosure of a copendingapplication filed in the names of Messrs. G. N. Saul and J. B.Shiel,-Serial No. 759,672, filed Dec. 29, 1934, and Fig. 4 correspondsto and is identical with Fig. 5 of the same copending application.

The eleven conductors extending to the left and the five conductorsextending to the right of Fig.

4 are not essential to an understanding of the present invention and maybe disregarded in the consideration of this application, they beingshown for convenience only as they are necessary for an understanding ofthe before mentioned Saul-Shiel application.

' A description of the operation of the switchboard circuits embodyingthe present invention will now be given. It will be understood that adistant operator has initiated a call on the toll line TL'of Fig. 1,which call has been brought to the attention of the toll operator havingcontrol of the cord circuit of Fig. 2 by the lighting of the usual linelamp, not shown. The operator 0 now inserts the rear or answering plug Iof the cord'circuit (Fig. 2) into jack 2 of the calling line. Thesupervisory lamp 3, however, does not light at this time due to the highresistance to ground through sleeve relay 4 of the calling line.

The operator now actuates. talking key 5,'thereby closing a circuit tooperate relay 6 which can be traced from ground at the right-hand outerback contacts of relay 1, over conductor 8, through right-hand upperoperated contacts of key 5, and left-hand winding of relay 6 to battery.Relay 6 in operating effects the transfer of certain functionalconductors from cord circuit TC to the common position circuit, Fig. 3,by which circuit they are controlled. The control of the beforementioned conductors will be described hereinafter in detail. D V

When relay 6 operated it closed a circuitfor operating relay 1 of Fig.3f;which maybe traced from battery through thewinding of relay 1,-

over conductor 9, through left and right-hand inner operated makecontactS.;,Qf "relay 6 to ground on the lower contacts of key 5. Relay 1in operating removes ground on its right-hand outer back contacts fromconductor 8, thereby preventing the: operation, in another cord cir--cuit of the position, of another relay corresponding to relay 6, if atalk key 5 of;-another cord circuit should be operated While key 5 ofthe tacts from sleeve conductor Hi and connects said conductor 10 toconductor II at its contacts 2, thereby completing, a circuit frombattery (Fig, 3) through the windings of relays l4 and l5 over conductorH to sleeve conductors l0, and thence through sleeve contacts of plu'gand jack2' and winding of relay 4 tocground. Relay |4 operates in thiscircuit, but relay |5 which isfmar'ginal' does not operate, at thistime. Relay 14in operating: operates relay l8 through a circuittraceable from battery through the winding of relay l8, right-hand breakcontacts of splitting key IB and relay 20, break contacts of relay l1.to ground on the operated contacts of relay |4. Relay l4 inoperatingalso closes a circuit from ground through the win'dingof relay=25, overconductor 2|, through operated contacts |3 of relay 6 to lamp 3 andbattery; Lamp 3, however, doesnot light due to the high resistance ofrelay 25. Relay |8- in operating operates relay. 26

through an .obvious circuit, and connects rear tip and ring conductors,21 and 28, 'with'its operated-.front-contacts to conductors 29inand 30-leading to the operators telephone circuit of Fig; 4.- Rel'ay 26 inoperating fdisconnects conductors 29 and 36 from a balancingterminationconsisting'of the series. combination of condenser 3|andgres'istance 32,the functioningof which termination being hereinafter described; J

- When relay 6 'operatedit closed a circuitfor I operatingzrelay 22,which may be traced from battery "through the winding of relay; 22,righthand operated inner make contacts of relay 6 to ground on thelower-contacts of talk key 5. Relay 22 in operating-connects with-itsupper and lower inner make contactstalkingconductors. 21: and 26120conductors and 34, which connect through; the normally; closed contactsor ringingkey 35 ,tq cord, circuit conductors- 36 and 31. The tolloperator may now converge over the calling line through the tip and ringcontacts of plug and jack 2., The aforesaid conductors29 and 30 connectto the right-hand terminals of winding 38 and 39 of induction coil 40.The left-hand terminals of said windings connect over conductors 3| and42 through lefthand and right-hand inner break contacts of relay 43 toanother balancing termination, consisting of a resistance 44 in serieswith a condenser 45, and balancing the impedance of the toll lineconnected to the right-hand terminals of windings 38 and 39. Theoperator's receiver 46 is connected for conversation to induction coilthrough a circuit traceable from the middle point of winding 38,right-hand break contacts of relay 5|, upper inner break contacts ofrelay 52', sleeve contacts of jack 4'! and plug 53,

receiver 46, sleeve contacts of plug 54 and jack 48, lower inner breakcontacts of relay 52, lefthand outer break contacts of, relay 5|,condenser 55 to middle of winding 39. The operators trans mitter 56 isenergized through a circuit traced from battery through retardation coil51, operated local contacts and tip contacts'of jack 48, tip contacts ofplug 54,transmitter 56, tip contacts of plug 53 and jack to ground 58.The voice currents generated by transmitter 56 are conducted to winding59 of induction coil 46 through a circuit in multiple, with thetransmitter. This circuit may be followed from conductor 6|], throughcondenser 6|, lower outer break contacts of relay 52, winding 59 ofinduction coil 40, upper outer breakcontacts of relay 52 to ground 58.The operators telephone arrangement just described isth'at ofthefaforesaid Dietze Patent 1,709,900 and described in said patent.

The operator having received instructions for it functions as completingthe call inserts plug 62 of the front cord into jack 63 of the calledline. This causes the operation of relay 23through a circuit that may betraced from battery through the windings of relays 23 and 24, overconductor 66, right-hand outer'operated make contacts. of relay6,.sleeve conductor 61 and sleeve'contacts of plug 62 and l jack 63.Relay 24 is marginal and does not operate at this time. The operation ofrelay I 23 operates relay 68, through'a circuit traceable from batterythrough the winding of relay 68,1eft-hand break contacts of splittingkey i9, right-hand 5 outer break contacts of relay 69 to ground on thecontacts'of relay 23. Relay 68 in operating operates relay 43 through anobvious circuit and connects talking conductors 10 and 1|, to conductors4| and 42, which connect to theinduction coil 40 I of the operatorstelephone circuit, as hereinbefore traced. The aforesaid operationofrelay 43 disconnects the hereinbefore mentioned balancing terminationconsisting of resistance 44 and condenser45; The operator now actuatesfront ringing key 64, which connects battery through resistance 65 tothe tip conductor of the calling cord through the upper outer makecontacts of key 64. This causes-an associated ringing circuit, notshown, to -function in a wellknown manner to connect ring-ing current tothe conductors of the called line;

When ringing key 64 was actuated a circuit was closed for operatingrelay 69 which may be 7 traced from ground through springs of key;64,

lower outer operated contacts of relayf22, over conductor 15, andthrough right-hand winding of relay 59 to battery. Relay 6!) inoperating operates slow releasing relay 16 from ground on conductor 15,through right hand operated inner contacts of relay 69 and winding ofrelay 15 to battery. Relay 16 in operating closesa circuit through theleft-hand winding of relay 69, lefthand inner operated contacts of relay6E! to ground on the contacts of relay 16.

According to a particular feature of this invention the arrangementdescribed in the preceding paragraph avoids the possibility of theoperator receiving disturbing clicks in her re- 10 ceiver from linedischarges after the ringing key is restored to normal after ringing.This is accomplished by the slow release characteristics of relay 75,which remains operated for an interval after ground is removed fromconductor due 15 to said restoration of ringing key 64. Relay Iii. holdsrelay 69 through its left-hand winding until it finally releases,thereby preventing the immediate reoperation of relay 6B and theconsequent immediate reconnection of conductors l5 and H to theoperators telephone circuit of Fig. 4. This delay is sufficient to allowthe calling line to dissipate the charge due to the ringing current, anddisturbing clicks in the operators receiver are accordingly avoided.

Let it be assumed now that the operator desires to ring on the rear oranswering cord. The invention now functions in a manner similar to thatdescribed for ringing on the front cord. Ringing key 35 is accordinglyactuated and battery through resistance 65 is connected by the upperouter make contacts of key to the tip conductor 35 of toll line TLthrough the tip contacts of plug I and jack 2. This causes theassociated-ringing circuit, not shown, to function. At

35 the same time key 35 also connects ground through its springs TI,lower middle make contacts of relay 22, over conductor '18 through theright-hand winding of relay 20 to battery. Relay 20 operates and opensthe operating circuit of 40 relay l8, which releases. Relay 26 alsoreleases and connects the balancing termination consisting of condenser3| and resistance 32 to conductors 29 and 30, thereby maintaining thebalanced condition of the operators telephone circuit.

When relay 2E3 operated it closed a circuit from ground on conductor 18,through its right-hand inner contacts and winding of slow releasingrelay 16 to battery. Relay '6 now functions in connection with relay 2!]in the same manner as hereinbefore described in connection with ringingon the front or calling cord, so that the line is discharged before theoperators telephone is reconnected following the restoration to normalof the ringing key. Objectionable clicks due to ringing are accordinglyavoided.

Let it be assumed now that the distant operator at the end of toll lineTL desires to recall the operator having control of cord circuit TC.Ringing current will be transmitted over the line and will operate relay19, removing ground from its back contacts and causing the release ofnormally operated relay 80. Relay 80 in releasing operates relay M whichconnects the winding of low resistance relay 82 in multiple with that ofhigh resistance relay 4. This reduces the resistance of the circuithereinbefore traced through the windings of relays l4 and i5, andmarginal relay l5 now operates and connects ground through resistance 83to conductor 2|, shunting the winding of relay 25 thereby causing thelighting of lamp 3 over the circuit before traced. If the distantoperator on the called line also desires to signal, a similar reductionin the resistance of the sleeve circuit of jack 63 is effected bycircuit arrangements not shown causing the operation of relay 24 and thelighting of lamp 84.

Let itbe assumed now that, the conversational connection beingestablished, and all cord circuit keys being normal the operator desiresto monitor on the connection. Monitor key 85 will accordingly beactuated and will connect ground through its lower outer make contactsto conductor 86, thence through the left-hand outer break contacts ofrelay 1 and winding of relay St to battery. Relay 5| operates andtransfers with its transfer contacts the circuit of the winding ofreceiver 46 from induction coil 53 to the left-hand winding ofmonitoring coil 83. The

right-hand windings of coil 88 are connected in series with condenser89, through inner break contacts of relay 1, break contacts 33 and 9! ofrelay 92, over conductors 93 and 9d, and the inner make contacts ofmonitor key 85 to the talking conductors of the cord circuit. Theoperator may now listen in on the connection.

If one of the talk keys of a cord circuit are operated, the operator mayoperate a monitoring key of another cord circuit and monitor on thatcord circuit. To describe this condition let it be assumed that the cordcircuit whose talk key has been operated has only its front cord pluggedup. Under this condition relay 1 is operated, due to the operation oftalking key 5, as hereinbefore described. The operation of relay itransfers with its transfer contacts, monitoring leads 93 and 94(connected to the cord circuit of which the monitoring key is operated)to leads S5 and 36, whichconnect to leads 2% and 30 through condensers9'! and 33. Leads iit and 3B connect to the right-hand terminals ofwindings 38 and 39 of induction coil 55. Conductor 86 leading to thewinding of monitoring relay 5i is also transferred by the left-handouter contacts of relay 1 through the left-hand outer break contacts ofrelay 92 to the winding of relay 26, which operates from ground on saidlead 85. The operation of relay 25 disconnects the balancing terminationconsisting of resistance 32 and condenser 3! from leads 2t and fit. Theleft-hand terminals of induction coil windings 38 and 39 are connectedover leads 4! and 52 and through operated contacts of relay 58 to leadsl0. and H, which connect to the talking conductors of the front cordthrough the operated contacts of relay 22. The toll operator may nowconverse and listen on both cord circuits simultaneously. If the rearcord of the cord circuit having the operated talk key 5 is plugged up,relay 25 is already operated when ground is connected to its winding bythe operation of the monitoring key 85 of the other cord circuit. Theoperator may monitor simultaneously on both cords as described before.

The invention was described hereinbefore for the conditions existingwhen the operator actuated the ringing key 35 or 64 on a cord circuitwith the talk key 5 of that cord circuit operated. It will now bedescribed for ringing with only a monitoring key 85 operated. Relay 92is now operated in a circuit from battery through the right-hand windingof relay 92, over condoctor 99, throughupper outer contacts of key 35 toground on the lower, outside operated contacts of front cord ringing key54, or rear cord key 35. Relay 92 in operating extends through itsright-hand, inner operated contacts the ground on conductor 99 to thewinding of slow releasing relay 16, which operates and holds relay 92through its left-hand inner contacts and winding. Relay 92 in operatingopens its break contacts 9 and 9|, thereby disconnecting monitoringleads 93 and 94 duringringing. The actuation of either of the aforesaidringing keys or 64 causes the associated ringing circuit (not shown) toring on the toll line, and to connect to the line not being called abalancing termination consisting of resistance 12 and condenser 13.After the restoration of the ringing key, relay 92 is held operated fora moment until relay'lii releases, thereby allowing the charge on theline to be dissipated before the monitoring leads 93 and 94 arereconnected to the cord circuit conductors by the'release of relay 92.

In the preceding paragraph the ringing function was described with amonitoring key operated on one cord circuit, and no keys operated on anyother cord circuit. Let it be assumed now that it is required to ring ona cord circuit Whose monitor key is operated, while a talk key 01another cord circuit is operated. Under this condition relay 1 isoperated as hereinbefore described, and the monitoring leads 93 and 94are accordingly connected to leads 95 and 96. When relay 92 now operatesin response to the actuation of the ringing key 35 or 64, thedisconnection of leads 93 and 94 is from leads 95 and 96. In otherrespects the operation is as described in the preceding paragraph.

If it is desired to ringon a cord circuit with no talk or monitoringkeys operated, the actuation of ringing 35 or 64 connects the balancingtermination consisting of resistance 12 and con denser 13 to the linethat is not being called.

To enable the toll operator to talk on either toll line separately,without being heard on'the other line, a splitting key I9 is provided.When the associated key lever is operated to the left, so as to talk onthe rear cord exclusively, the operating circuit for relay 68 is opened;Relay 68 in releasing disconnects front cord conductors 10 and H fromleads 4| and 42 to the operators telephone circuit, andwhen relay 43also obviously releases, the front balancing termination is connected toleads 4| and 42.- Inorder to balance the disconnected line-associatedwith the front cord, the operation of key I9 closes a circuit forconnecting the balancing termination consisting of resistance 32 andcondenser 3! to the front cord talking conductors l0 and H. This circuitmay be traced from conductor 10, through lefthand outer break contactsof relays 68 and 69, left-hand upper make contacts of key l9, condenser3|, resistance 32, left-hand lower make contacts of key l9, andright-hand break contacts of relay 68, to conductor ll. With thisarrangement the operator is enabled to talk on the line associated withthe rear cord, without being heard by the line associated with the frontcord. In like manner, if splitting key I9 is operated to the right orfront position, the operator may converse on the front cord only. Inthis case relays I8 and 26 release and the termination consisting ofresistance 44 and condenser 45 is connected through the contacts of keyl9 to the talking conductors 21 and 28 associated with the disconnectedrear cord line.

The invention has been described for the usual functions involved inestablishing a conversational connection between two toll lines,including ringing, splitting and supervision, and the operation of theoperators telephone circuit for talking and monitoring. It will now bedescribed for certain additional features, to which it is desired thatspecial attention be given.

, Let it be assumed now that the toll operator in control of cordcircuit TC (Fig. 2) has observed theactuation of the line signal (notshown) associated with toll line TL (Fig. 1) and operatesher talk key 5before inserting plug I into jack 2. Without the particular features'ofthis invention, which will now be described, it is to be expected thatthe operator will frequently hear in her receiver disturbing clicks dueto the ringing current still connected to the toll line at the time ofplugging in. Due to the operation of talk key 5 relay 1 is operated asbefore described. Slow release relay H operates through a circuittraceable from battery through the winding of relay l1, break contactsof relay 25 to ground on the right-hand outer operated contacts; ofrelay 1. Relay I! in operating opens withits break contacts theoperating circuit for relay I 8. When plug l is inserted into jack 2, ifringing'relay I9 is operated the sleeve resistance of jack 2 will be lowand both relays I4 and l5'will operate. Under this condition relay 25cannot operate due to the shunting effect of resistance 83- connected toground to the contacts of relay l5. Relay ll accordingly remainsoperated and keeps open the operating circuit of relay I8, which in notoperating does not connect talking conductors 2'! and 28 to theoperators telephone circuit. Accordingly ringing current is kept out ofthe operators receiver. When, however, ringing current ceases, relay l5releases, removing shunt 83 and relay 25 operates, through a circuitfrom battery through lamp 3, operated contacts l3 of relay 6, overconductor 2|, through winding of relay 25 to ground on the operatedcontacts of relay I 4. Relay '25 I in operating releases relay [1, whichclosesthe circuit of relay l8 which operates, and connects leads 2'! and28 to'the operators telephone circuit, thereby enabling the operator toconverse on th calling line.

Another feature of the invention will now be described wherebyanarrangement is provided to enable a supervisor to communicate with thetoll operator by plugging in her telephone set in multiple with that ofthe operator when the cord circuit talk keys-are normal. As hereinbeforedescribed, the operators telephone circuit, when. no talk key is'operated, is balanced by the connection of a termination consisting ofresistance 44' and condenser 45- to the left-hand terminals of windings38 and 39 of induction coil 40, and of a termination consisting ofresistance 32 and condenser 3| to the corresponding right-hand terminalsof said winding. Under this balanced condition a telephone set inmultiple with that;

of the toll. operator would be ineffectivedu'e to the anti-sidetonecondition. When however, a supervisor plugs into jacks 49 and 59 with atalephoneset,.not shown, but identical with that convsisting of receiver46 and transmitter 56 this invention provides an arrangement whereby theaforementioned. balanced condition is removed, enabling supervisor andoperator to converse. When'plugs are inserted into jacks 41 and 49 acircuit is closed for operating relay 25, which maybe traced frombattery through the winding of relay 26, operated local. contacts ofjacks 41 and 49, over normally closed contacts of a grouping key shownin Fig. 5 of'the before-mentioned copending application of Saul'andShiel, Serial sistance 32 and condenser 3|, and accordingly, due to theunbalance resulting, the anti-side condition is removed and supervisorand operator may converse. If the operator should now operate the talkkey 5 of a cord circuit and insert plug 62 into jack 63, relay 43 wouldoperate as before described, releasing relay 26, which would reconnectthe rear termination consisting of said resistance 32 and condenser 3 Ireestablishing the normal anti-sidetone condition.

What is claimed is:

1. In a telephone system, a link circuit, an operators telephone circuitincluding a transformer having two windings, means for seriallyconnecting each winding in a different conductor of said link, animpedance network normally connected across both corresponding ends ofthe two windings, an operators receiver connected between intermediatepoints of the two windings, a second receiver, means for connecting thesecond receiver in multiple with the operators receiver, and meansresponsive to the connection of said receivers in multiples todisconnect one of said networks from the transformer.

2. In an operators telephone circuit, a hybrid induction coil, atransmitter and receiver connected thereto, said coil being normally sobalanced with respect to the connected transmitter and receiver thatsounds afiecting the transmitter cause no corresponding effect in thereceiver, a second transmitter and receiver, means for connecting thesecond transmitter and receiver to the induction coil in multiple withthe first transmitter and receiver, and means responsive to saidmultiple connection to unbalance said coil.

' 3. In a telephone system, an operators cord circuit, a line connectedthereto, an operators telephone circuit connected to the cord circuit, asource of ringing current, ringing means for connecting said source tothe cord and then to the line, means responsive to the actuationof saidringing means for disconnecting said operators telephone circuit fromthe cord, and other means for delaying the reconnection of saidtelephone circuit to the cord for a predetermined interval after saidringing current has been disconnected.

4. In a telephone system, an operators'cord circuit, a line connectedthereto, an operators telephone circuit connected to the cord circuit, a

source of ringing current, ringing means for connecting said ringingcurrent to said cord and then to said line, means actuated when saidringing current is connected to the cord to disconnect said operatorstelephone circuit from the cord, and means for delaying the reconnectionof said telephone circuit to the cord for a predetermined interval aftersaid ringing current has been disconnected.

5. In a telephone system, an operators cord circuit, a line connectedthereto, an operators telephone circuit connected to'the cord circuit, asource of ringing current, means associated with the cord circuit forcausing said source of ringing current to be connected to the line,means responsive to the actuation of said ringing means fordisconnecting said operators telephone circuit from the cord, and othermeans for delaying the reconnection of said telephone circuit to thecord for a. predetermined interval after said ringing current has beendisconnected.

6. In an operators telephone circuit, a hybrid induction coil, atransmitter and receiver connected thereto, a balancing impedanceconnected to one terminal end of said induction coil,

a balancing impedance connected to the other terminal end of saidinduction coil, the connection of said balancing impedances causing saidinduction coil to be so balanced that sounds affecting the transmittercause no corresponding efiect in the receiver, a second transmitter andreceiver, means for connecting the second transmitter and receiver tothe induction coil in multiple with the first transmitter and receiver,and means responsive to said multiple connection for disconnecting oneof said balancing impedances so that said induction coil is therebyunbalanced and sounds affecting either or both of said transmitters arereproduced in both of said receivers.

GEORGE N. SAUL.

